1. Field of the Invention
This invention relates to demodulators, and more particularly to a novel demodulator for a signal which is phase-modulated so that the time average of its frequency is equal to a carrier frequency. The signal is converted into an intermediate frequency signal to keep the intermediate frequency stabilized and to permit easy demodulated to the original digital signal.
2. Description of the Art
So far, the PSK (phase shift keying) modulation system, in which a digital signal is transmitted with the bits "0" and "1" in correspondense to the phases of the carrier, is extensively employed for transmission of digital signals, because of its good characteristics to noise.
The PSK modulation system will be described in more detail.
There are a lot of PSK modulation techniques. On of them which is frequently used is MSK modulation. In MSK modulation, the phase of a carrier is linearly increased by 180.degree. for one time slot of the signal when a digital signal is in a "mark" state, and the phase is linearly decreased by 180.degree. for one time slot of the signal when a digital signal is in a "space" state.
Another modulation technique is a so-called "DSK" technique, in which, as shown in FIG. 6, one time slot is divided into two equal parts and the phase of a carrier is increased by two steps, namely 90.degree. each, for the one time slot for a "mark" state of a digital signal, the phase is decreased also by two steps, 90.degree. each, for a "space" state of a digital signal.
The MSK system is advantageous in that, as the phase is changed linearly, the occupied frequency bandwidth is narrow. The DSK system is advantageous in that it, having good characteristics under condition of multi-path fading, is suitable for wide-band and high speed data transmission.
There are two commonly used demodulation techniques for PSK-modulated signals, synchronous detection and delay detection.
In the delay detection, signals received are divided into two parts. One is supplied to a phase comparator after being delayed by appropriate time duration with a delay circuit, while the other is supplied to the phase comparator as it is, so as to demodulate the PSK-modulated signal thereby to obtain the origianl digital signal.
This will be described in more detail. It is assumed that, in a delay detector shown in FIG. 3(A), its input voltage Vin is represented by the following expression: EQU Vin=cos(.OMEGA.t+.theta.(t))
Where .OMEGA. is the angular frequency of a carrier, t is the time, and .theta.(t) is the phase modulation function. The input voltage Vin is divided into two ways parts. One of the two parts is applied to one input terminal of a phase comparator 22, while the other is applied to the other input terminal of the comparator 22 after being delayed by a predetermined period of time TR. Therefore, the signal Vc applied to the one input terminal is: EQU Vc=Vin=cos (.OMEGA.t+.theta.(t))
and the signal Vd applied to the other input terminal is: EQU Vd=cos (.OMEGA.(t TR)+.theta.(t-TR))
Where the phase comparator 22 is designed as shown in FIG. 3 (B) and provides an output proportional to a phase difference as shown in FIG. 3(C), the phase difference .DELTA..theta. is: EQU .DELTA..theta.=TR +.theta.(t)-.theta.(t-TR)
The delay time TR should meet TR=T/2 (where T is one time slot of the signal) in the MSK system or DSK system.
With .OMEGA. TR=(2n-1), namely with .OMEGA.=(2n-1).pi./TR =(2n-1)2.pi./ T, the reference point for phase comparison can be set at the center of the range of operation of the phase comparator.
The operation of the DSK modulation will be described by way of example; however, it should be noted that the description is applicable to the MSK system as well. EQU In the case of .theta.(t)-.theta.(t TR)=0,
The operation reference point of the phase comparator is expressed as EQU .DELTA..theta.=.OMEGA.TR-(2n-1).pi.
Therefore, the output of the phase comparator will be the one corresponding to the operation point which is shifted as much as .theta.(t)-.theta.(t-TR) from the reference point.
In the case where the signal is of "mark" followed by "space" , the phase function .theta.(t) is as shown in FIG. 4 (A) and .theta.(t-T/2) is as shown in FIG. 4(B).
Accordingly, .theta.(t)-.theta.(t-T/2), as shown in FIG. 4(C) is .pi./2 for a "mark" period and -.pi./2 for a "space" period, and an output waveform as shown in FIG. 4(E) is obtained according to an output characteristic as shown in FIG. 4(D). That is, the output is 3V.sub.0 /4 for the "mark" period, and V.sub.0 /4 for the "space" period.
Accordingly, when the output of the phase comparator 22 exceeds V.sub.0 /2, the signal is judged as "mark". When the output is lower than V.sub.0 /2, the signal is in a "space" state.
On the other hand, when a synchronous detection circuit is used for demodulation, a signal received is divided into two parts, which are applied to two phase comparators, respectively. An output signal of a voltage-controlled oscillator in a phase locked loop(whose frequency is coincident with the carrier frequency of the signal received) is applied to one of the phase comparators, and the output signal is supplied to the other phase comparator with its phase shifted by 90.degree., so that the original digital signal is obtained from the output signals of the two phase comparators (cf.Trans.IECE Japan, Vol. 64-B, No. 10, 1981, GMSK Modulation System Transmission Characteristic by Kazuaki Murota and Kenkichi Hiraide).
In demodulating the PSK-modulated signal according to the above-described delay detection system, the signal received is divided into two parts, one of which is merely delayed. This has the advantage that the circuitry can be simplified. However, if this method is applied to the transmission of digital signals in a high frequency band, the demodulation reliability is lowered.
This will be described in more detail. In the delay detection system, the operating reference point is .DELTA..theta.=.OMEGA.T/2. Therefore, if the carrier angular frequency drifts by .DELTA..OMEGA., for instance, by a temperature change, then the operating reference point will be changed by .DELTA..OMEGA.T/2. If this change is large, then it is difficult to determine the "mark" and "space" according to whether or not the output level of the phase comparator exceeds V.sub.0 /2. For instance when the carrier frequency is 2.5 GHz, and the temperature variation of the oscillator (such as a saw tooth wave oscillator) is .+-.3.times.10.sup.-4, then the frequency variation will be of .+-.750 KHz. If, in this case, the data transmission speed is set to 32K bps, then T=1/32 msec, and .DELTA..OMEGA.T/2=23.44 .pi.; that is, the drift of the operating reference point is about substantially 23.44 .pi.. In practice, the operating reference point, being affected by noise, and interference waves coming through multiple paths in addition to the temperature variation, is further shifted. Therefore, it is difficult to determine the "mark" and "space" through comparison of the output level of the phase comparator with a predetermined reference value.
The synchronous detection system described above is based on the reproduction of a carrier frequency by a COSTAS loop. In this system, unlike the phase delay detection system, the difficulty due to the frequency variation never takes place, and the signal can be demodulated with high accuracy.
However, the synchronous detection system has its own disadvantages. It is necessary to provide a voltage-controlled oscillator as a local oscillator and a phase locked loop to obtain the signal whose frequency is equal to the carrier frequency of a signal received. This requirement will make the circuitry intricate and increase the manufacturing cost. This problem is a serious matter especially for mobile radio equipment because of the requirement for a miniaturization and simplification of the mobile radio equipment and for a reduced manufacturing cost.
The present inventor has proposed (in Ser. No. 072,162 filed July 10, 1987, the disclosure of which is incorporated herein by reference) system in which the delay time is made equal to the total time of reference phase parts of PSK-modulation signal according to an NRZ signal obtained after demodulation of the modulated signal in order to reduce the total time of the reference phase parts, and the variation .DELTA..theta..DELTA.T of the operating reference point is thereby decreased to improve stability.
While this proposal is advantageous, if it is employed in a system where high frequencies are used for carrier frequencies, the improvement in stability is limited, and the degree of technical difficulty is increased.
This will be described in more detail. If the total time of the reference phase parts is decreased, then the rate of variation of phase to time is increased, and the occupied frequency band width of the modulation wave is increased. Furthermore, steep pulse waves must be handled in the signal (e.g. video) processing stage after detection; that is, high frequency components must be processed. This increases the degree of technical difficulty and increases costs.
The inventor has also proposed (in Ser. No. 076,173 filed July 21, 1987, the disclosure of which is incorporated herein by reference) two other PSK modem systems in which demodulation is based on the time average of the instantaneous angular frequencies of two respective kinds of PSK modulation waves. In one of the other systems, the PSK modulation wave has reference phase parts whose total time is a predetermined value provided at the front and/or rear part of one time slot of a digital pulse signal, and in the front half of the remaining part of the time slot, the phase is changed in a predetermined direction in correspondence to one of the "mark" and "space" states of a transmission signal, and in the rear half, the phase is restored to the reference phase. For the other state of the transmission signal the phase changes in the opposite direction front that for the one state. In the second one of the other systems a PSK modulation wave is used in which the phase of the transmission signal for one of the "mark" and "space" states is as described above for the first system; however, only the reference phase part is formed over the entire range of one time slot of the digital pulse signal in correspondence to the other state of the transmission signal. In such systems the time average of the instantaneous angular frequencies is equal to the carrier angular frequency. A local oscillation frequency to be mixed with the received signal is feedback-controlled by the utilization of a signal which is obtained by subjecting an intermediate frequency signal to frequency detection, thereby to stabilize the carrier wave angular frequency of the intermediate frequency signal, and the signal received is demodulated by the utilization of the signal thus stabilized.
In such systems, a detector having a linear frequency response is, in general, employed as a frequency detector for subjecting the intermediate frequency signal to frequency detection. The linear detector, being made up of analog circuits, has a relatively large fluctuation characteristic. Accordingly, it is essential to adjust each of the demodulators to allow the latter to operate as required. Especially in a mass production of the demodulators, the time and labor required for such adjustment will increase the manufacturing cost.